Answer
Please refer to the step-by-step part below.
Work Step by Step
Use the identity $\cos{(\alpha+\beta)}=\cos{(\alpha)}\cos{(\beta)}-\sin{(\alpha)}\sin{(\beta)}$ to obtain:
$\cos{(\frac{\pi}{2}+\theta)}\\=\cos{(\frac{\pi}{2})}\cos{(\theta)}-\sin{(\frac{\pi}{2})}\sin{(\theta)}\\=0\cos{(\theta)}-1\cdot \sin{(\theta)}\\=-\sin{(\theta)}.$